1. Field of the Invention
This invention relates generally to closure systems and, more particularly, to a screw type closure which is useful in any closure application, but is particularly useful as a closure for wide mouth containers.
2. Description of the Prior Act
Screw caps or closures fall into two general categories. In the first class the caps have a continuous internal thread which cooperates with a corresponding external thread formed on the external surface of a neck portion or finish of a container. In the second class, caps have a plurality of spaced lugs or have interrupted threads, formed on the internal surface thereof, which cooperate with corresponding cams or interrupted thread segments formed on the external surface of the neck portion or finish of the container. The first class is by far the most common. Examples of the second class are found in U.S. Pat. Nos. 1,612,449 and 4,202,462.
In addition to the two general classes discussed above there are hybrids which use spaced interrupted threads on a closure in combination with continuous threads on a neck portion of a container, or vice versa. U.S. Pat. No. 1,783,314 discloses a closure with interrupted threads for use with a container having continuous threads on the neck portion.
Lug caps, when used with containers with corresponding cam segments, are usually easier to apply and remove than continuous thread caps. One or more complete turns may be required to remove the continuous thread cap from a container, while a fraction of a turn will apply and remove most lug caps. Many lug caps are formed from metal, but metal has problems with corrosion and paint or laquer scratching on the surface. Moreover, such metal caps tend to be more expensive.
A typical unscrewing closure which is molded from synthetic plastic material consists of three main parts--a top wall, a skirt or side wall depending from the top wall and continuous threads formed on the interior wall of the skirt. The top wall, in combination with any desired additional sealing means such as a liner or gasket provides the necessary seal to protect the product in the container from the ambient environment. The threads provide the capability of applying force to hold the closure in its sealing position on the container. The skirt acts as a link between the threads and the top wall.
In designing a mold for manufacturing a conventional unscrewing closure the following factors are important. The number of threads formed on a closure skirt are limited because of the travel required by an unscrewing mechanism associated with the mold to remove a closure from the mold. Cooling in the core of the mold is limited due to the gear size. Unless core venting is provided, wide mouth closures frequently have sunken top walls.
Controlling the above-noted factors reduces the productivity that might be otherwise achieved. A collapsible core has been used as one way of coping with these factors. However, tooling cost, size limitation and the cooling problems still remain.